‘Adolescence is the age when you’re in a turmoil because you’re trying to work out who you are and gender is a big part of that.

However, other medical experts have questioned the safety of the treatment, warning that little is known about its long-term mental, psychological and physical effects.

Last month three top US doctors, Professors Paul Hruz, Paul McHugh and Lawrence Mayer, published a highly critical report on the use of puberty-blockers to treat gender dysphoria.

Writing in American academic journal The New Atlantis, they warned that the safety of this ‘experimental’ treatment was ‘unsupported by rigorous scientific evidence’.

They further argued that the use of such drugs may be driving children to ‘persist in identifying as transgender’.

Research shows that the vast majority of under-16s who are troubled about their gender do not go on to take the drastic step of surgery.

Meanwhile, the three professors point to another study from a Dutch clinic – where all the adolescents prescribed puberty blockers had gone ahead with gender-reassignment surgery – as evidence that

Concern has also been raised about the blockers’ long-term impact on bone health.

But Prof Butler said the drugs have no ‘permanent effects’ on the reproductive system or the body as a whole.

‘When you stop the blocker, the puberty hormone process just starts up again within a couple of months. If you’ve gone through puberty already, you start where you left off,’ he said.

‘You don’t go back to the beginning again. If you haven’t gone through puberty, you would just complete the full puberty development process.

‘You would go through it at the same rate as you would if you hadn’t taken the blockers, but the timing will just have been delayed.’

He also insisted that the drugs were safe because they had already been used for decades to treat other conditions such as fertility problems in women and prostate cancer.

Addressing fears about the effects the drugs can have on bone development, Prof Butler said there was no need to ‘worry unduly’.

‘Our preliminary analysis suggests that the blocker just halts bone-calcium increases – it doesn’t weaken the bones directly,’ he added.

Prof Butler said his gender identity clinic now ‘routinely’ prescribes puberty-blockers to children diagnosed with ‘life-long’ gender dysphoria – the belief that a person is inhabiting the wrong sex.

‘When the team feels the young person is genuinely transgender they welcome [the use of] puberty-blocking drugs right from the early stages,’ he said.

His clinic has treated more than 600 under-18s with the blockers and Leeds Gender Identity Service has prescribed them to a further 200, he said.

Of these, he said ‘about 230’ were 14 or under – with the youngest being ten.

His disclosure comes three years after The Mail on Sunday revealed how NHS doctors were set to give the puberty blockers to nine-year-olds – causing outcry from critics who accused them of ‘playing God’ with children’s lives.

Explaining the process, Prof Butler said his patients were first ‘carefully’ psychologically assessed by experts at the NHS’s nearby Tavistock Clinic.

‘We then do medical assessments to see if the young person has started puberty and how far they are into puberty,’ Prof Butler said.

If suitable, both physically and psychologically, they then advise GPs to prescribe the blockers.

He added: ‘We’re lucky in the UK that people don’t miss out – they will get this treatment.’

Advocates of puberty-blockers argue that it represents a prudent and ‘fully reversible’ way to give young people with gender dysphoria and their families time to sort out the difficult issues surrounding gender identity.

Puberty-suppression as an intervention for gender dysphoria has been accepted so rapidly by much of the medical community, apparently without scientific scrutiny, that there is reason to be concerned about the welfare of children who are receiving it.

There remains little evidence that puberty-suppression is reversible, safe, or effective for treating gender dysphoria.

Psychologists do not understand what causes gender dysphoria in children and adolescents.

They also cannot distinguish reliably between children who will only temporarily express feelings of being the opposite sex from children whose gender dysphoria will be more persistent.

We frequently hear from neuroscientists that the adolescent brain is too immature to make reliably rational decisions.

But we are supposed to expect emotionally troubled adolescents to make decisions about their gender identities and about serious medical treatments at the age of 12 or younger.

For patients and doctors who are committed to the view that the young person’s gender dysphoria represents a persistent and real problem that can best be solved by transitioning the patient to living as the opposite sex, puberty-suppression can seem like a desirable approach.

But most children who identify as the opposite sex will eventually come to identify as their biological sex.

Until much more is known about gender dysphoria, and until controlled clinical trials of puberty-suppression are carried out, this intervention should be considered experimental.

Regardless of the good intentions of the physicians and parents, to expose young people to such treatments is to endanger them.